1. Field of the Invention
This invention relates to a system and method for performance of a Built-In Test (BIT) for an optical transceiver in which a transmitter emits a collimated optical beam and a receiver detects optical radiation in fields of view that overlap at a target.
2. Description of the Related Art
An optical transceiver includes both a transmitter and a receiver on the same platform. The transmitter emits an optical beam within a field-of-view (FOV) and the receiver detects optical radiation within a field-of-view (FOV). The optical transmit beam has a center wavelength from the UV band of approximately 0.3 microns to the LWIR band of approximately 12 microns. The transmit beam is “collimated” if the RMS deviation of its wavefront from a perfect plane wave is less than or equal to ¼ wavelength, preferably 1/10 wavelength or less. The transmit beam is often polarized but may be unpolarized. For example, laser sources are typically linearly polarized. The receiver may comprise a single detection element, a quad-detector or a large array of detection elements and may be imaging or non-imaging. The transmitter and receiver are aligned, either co-boresighted or boresight offset, so that their fields of view overlap at a target range. The transmitter and receiver may be fixed or gimbaled to slew over a field-of-regard (FOR).
The transmitter and receiver may operate in tandem or independently. The transmit beam may “paint” the target to produce a return that is detected by the receiver. The return signature may be processed to, for example, acquire the target, determining a bearing or range to target, identify the target, select an aimpoint on the target etc. Examples include LIDAR systems, active optical target detection (AOTD), active seeker and a laser range finder. Alternately, the receiver may detect a passive signature emitted by the target to acquire and track the target. For example, the receiver may detect the IR heat signature of a missile. The transmit beam illuminates the target to, for example, jam the missile seeker. Examples include a directed infrared countermeasures (DIRCM) system for military or commercial aircraft.
A Built-In Test (BIT) is used to ensure the operability of the optical transceiver. The current state-of-the-art is to simply detect the flash produced by backscatter of the transmit beam off of the transmitter optics. If a flash is detected the transmit source is deemed to be operational. Typically, a separate BIT detector is used to detect the “flash”. In a gimballed co-boresighted DIRCM system such as Northrop Gruman's AN/AAQ-24(V) Nemesis, the “eye” can rotate a full 180 degrees. This is done primarily for storage to protect the optics but may also be used to perform a BIT. The eye is rotated 180 degrees so that the transmit beam is directed straight back into the detector. If the detector is on-gimbal, a separate BIT detector may be positioned to detect the transmit beam.